Stereospecific [3H]sufentanil binding, inhibited by dextromoramide, represents 90% of the total binding in membrane preparations of rat brain and spinal cord. Scatchard plots of the binding in the forebrain, at 37 degrees C in Tris-HCl buffer without and with 120 mM NaCl, were rectilinear; KD = 0.13 nM and 0.31 nM, Bmax = 13 fmol/mg tissue and 9.9 fmol/mg tissue in the absence and the presence of sodium ions respectively. The reduction in binding affinity in the presence of sodium ions was found to be due to a 9.7 fold enhancement of the initial dissociation rate from t1/2 = 2.1 min in the absence to 13 s in the presence of sodium ions. The [3H]sufentanil binding properties were superior to those of [3H]fentanyl, [3H]dihydromorphine and [3H]naloxone; [3H]sufentanil showed an unmatched favourable ratio of stereospecific versus non-specific binding; it had a 7.7, 20 and 40 fold binding affinity than the above ligands respectively. Due to its relatively slow dissociation rate, a more accurate estimation of the Bmax value was obtained with [3H]sufentanil than with the other, fast dissociating 3H-ligands (t1/2 less than 10 s). A total of 37 narcotic analgesic agonists and antagonists belonging to 5 different major structural classes all inhibited stereospecific [3H]sufentanil binding in a competitive way. There was no relationship between binding affinities and lipophilicity and degree of ionization of the compounds. Binding affinities correlated highly significantly with the analgesic potency measured in vivo, demonstrating that [3H]sufentanil labels mu-opiate receptor sites which mediate narcotic analgesia. Moreover, the binding affinity of sufentanil for delta-type binding sites labelled by [3H] [D-Ala2,D-Leu5]enkephalin was found to be 100 times lower than its binding affinity for the mu-receptor sites. [3H]Sufentanil was used for a detailed investigation of the regional distribution of mu-opiate receptor sites in the brain; Bmax and KD values were measured in the dorsal and ventral spinal cord.